These are terms of the trade. Your opinion is interesting to me, but won't have much of an effect on ranchers and butchers.
"Lamb" means a year old or less.
"Spring lamb" means unweaned ("milk-fed,") and FWIW they can be slaughtered into the early summer. I forget where the line is, late June or mid July I think.
"Baby lamb" is milk fed and no older than 8 weeks, IIRC.
I've done a few lambs, here are some impressions.
If Sam's weights are dressed weight, figure a little more than 50% waste to bone, moisture loss etc. You're not going to lose much to fat on the younger lambs -- but a little more to the older. If Sam's weights are live weight figure more about 2/3 to waste. A 25 pound (dressed) lamb will feed about 16, if there's plenty of other food going on. If you're looking to mostly eat saddle (legs and chops), it's big enough for eight. I'd definitely check with Sam on whether the weight means on the hoof or dressed, and what the weight difference is between the two. I'd also check with her and/or another independent source on portion size. I have a slight tendency to overfeed.
Lamb chops from baby lambs are called chuletillas in Spain. Big deal luxury food there.
The best way to cook lamb is over a live fire. "Caja China" don't really do an interesting job. What they do allow for is very humid cooking. Great for Cuban style and Hawaiian pig. Otherwise, big waste of time.
Lamb loves smoke -- heck, it's already got a nice smokiness to it. The preferred fire in the west is red oak burnt down to coals -- and that's my highest recommendation. In your neck of the world, if I couldn't get red oak, I'd look for white oak as a second choice -- than any other fruitwood or pecan. Grape cuttings are awesome, especially mixed with oak. I find hickory brings more association with pork than I'd like, but your guys may want it. Personally, I'd take mesquite over hickory.
The top European wood choices are oak, cherry, citrus and grape -- in no particular order.
If you can't deal with a hardwood fire, at least use hardwood lump charcoal. Friends don't let friends use briquette. Or lighter fluid for that matter.
A spit is very nice. Depending on the size of the pit over which you've got the spit, you probably want a fairly moderate fire.
But California style you want to keep the fire very hot, but get the lamb far away enough away from it so that it's cooked by a mixture of radiant heat and convecting hot air -- and far away from any flare ups. You're looking for a grill temp in the neighborhood of 325F. This is sort of a cross between "open pit" and "California" aka "Santa Maria" style. You could call it either. Out here we have special open pits with adjustable height food grates or coal grates. You've probably seen pictures of big grills with wheels that raise and lower the grates on chain. We control the temp by changing the distance between food and fire.
Regular smokers big enough to handle the food are good as well. It's so easy, it's almost cheating. I hate to mention it.
If you go spit or California AND hardwood, bring along a second pit -- a Weber kettle will do, so you can keep making coals to feed your cooking fire. Also, keep a bowl of water by the fire with a lot of fresh rosemary soaking. Throw some on the fire now and then. Also a few heads of garlic.
Best way to handle lamb open-pit or in the smoker is to butterfly it, stretch it on metal stakes, and wrap the whole thing in chicken wire (makes it easier to turn). You'll only turn it once in the smoker; and more than a few times if you go California style.
If you want great texture and great taste, you've got to go spit or open pit. You know you want that good grill crispness and char. You really, really want that.